1. Field of the Invention
The present invention relates to a linear drive unit having a linear motor provided with a coil and a magnet, and relates to a machine tool having the linear drive unit.
2. Description of the Related Art
FIG. 5 schematically shows a structural example of a conventional linear motor. A linear motor 100 has a stator 106 formed by locating magnets 102 on an magnet core 104, and a moving element (or a coil) 112 formed by winding a three-phase winding wire 110 on a coil core 108, wherein a thrust force in a direction indicated by an arrow 114 is generated by current passing through winding wire 110. In general, the coil side is configured as a movable part, and the magnet side is configured as a fixed part. Magnets are aligned in the thrust force direction so that N-poles and S-poles are alternately positioned, over a length which is a summation of the length of the coil in the thrust force direction and a movable range (or a stroke length) of the coil.
FIG. 6 shows a schematic configuration of a linear drive unit 120 of the prior art, on which the conventional linear motor as shown in FIG. 5 is mounted, viewed in the thrust force direction of the linear motor. Coil 112 is fixed to a slide (or a movable part) 122, and stator 106 is fixed to a base (or a fixed part) 124. Slide 122 is supported by base 124 via a bearing 126 so that the slide is movable in the thrust force direction only. Therefore, in linear drive unit 120, a distance (or a magnetic gap “g”) between coil 112 and stator 106 (magnet 102) is previously determined as a constant value.
The magnetic gap is an important factor which affects the performance of the linear motor. When the magnitude of the magnetic gap is changed, the performance (or the thrust force) of the linear motor is largely varied. As the related art document, JP 2003-250258 A discloses a linear motor wherein a moving element 12 is configured to slide relative to a movable part (or a table 2) in the vertical direction.
In the conventional linear motor as shown FIG. 5, it is noted that the thrust force is largely varied due to a change in the distance (magnetic gap “g”) between coil 112 and magnet 102, even when the magnitude of current in coil 112 is not changed. Further, in the linear motor having coil core 108 as shown in FIG. 5, a magnetic flux density of magnet 102 is different by location, and thus a magnetic attractive force generated between coil 108 and magnet 102 when driving the motor is varied (i.e., cogging occurs). As magnetic gap “g” is reduced, the thrust force is increased and cogging is also increased. On the other hand, as magnetic gap “g” is increased, the thrust force is lowered and cogging is also lowered.
In a linear motor used for an ultra-precision processing machine, etc., in which a high feeding accuracy is required, in order to avoid deterioration in straightness of a feed axis due to cogging, the magnetic gap is set to a relatively large value so as to lower the thrust force. Otherwise, by using a coil having no core (i.e., a coreless coil), cogging cannot be generated in principle.
However, when the magnetic gap is increased or when the coreless coil is used, the cogging is limited whereas the thrust force of the linear motor is lowered. Therefore, in order to obtain large thrust force, it is necessary to apply large current to the coil. In this case, due to generated heat of the coil, machine accuracy of the linear motor or the machine tool having the linear motor may be deteriorated. In general, since a heat capacity of the machine tool is relatively high, it takes time for the temperature of the machine tool to return to normal after the temperature is changed (increased). Further, as one measure for avoiding the above problem, a cooling mechanism may be incorporated in the machine tool. However, since it is impossible for the position of the cooling mechanism to completely coincide with the position of the heat source, temperature distribution is inevitably generated in the machine tool.
In the invention of JP 2003-250258 A, moving element 12 is configured to slide relative to table 2. This is intended that thickness D of stator 11 and thickness R of moving element 12 are negligible when adjusting the distance (or the air gap) between stator 11 and moving element 12, the influence of relatively heavy table 2, and the positional relationship between table 2 and machine base 1 in the vertical direction even when moving element 12 is vibrated during use. In other words, in JP 2003-250258 A, the magnitude of the air gap is determined by second rail 7 and second slide 9, and it is not suggested that the magnitude of the air gap is purposely changed.